While fabricating semiconductor lasers or other electronic devices, it is required to diffuse n type impurities into group III-V compound semiconductors. Conventionally, silicon is generally used as an n type impurity to be diffused into group III-V compound semiconductors. However, diffusing silicon into a semiconductor in a large quantity causes technical difficulties in many cases.
On the other hand, in fabricating AlGaAs series semiconductor lasers, a technique of diffusing an impurity from a solid phase source, i.e. using a selenium doped semiconductor layer as a diffusion source has been developed for disordering a superlattice layer.
FIGS. 3(a) and 3(b) show a method for disordering a superlattice structure comprising AlGaAs series material as the group III-V compound semiconductor by diffusing selenium, which is disclosed in Japanese Published Patent Application 1-143285. In FIGS. 3(a) and 3(b), a first AlGaAs layer 4 is disposed on a GaAs substrate 11 and a superlattice layer 7 comprising AlGaAs/GaAs is disposed on AlGaAs layer 4. A second AlGaAs layer 9 is disposed on superlattice layer 7 and a selenium doped GaAs layer 2 is disposed on AlGaAs layer 9. A selenium diffusion region 10 is produced in the layers 9, 7 and 4 and the diffusion region 8 in the superlattice layer 7 becomes disordered.
A description is given of the diffusion process.
First of all, a first AlGaAs layer 4, a superlattice layer 7 comprising AlGaAs/GaAs, and a second AlGaAs layer 9 are successively epitaxially grown on GaAs substrate 11 by liquid phase epitaxy (LPE) or metal organic chemical vapor deposition (MOCVD) and further a GaAs layer 2 containing selenium and doped to an order of 10.sup.18 cm.sup.-3 is epitaxially grown on second AlGaAs layer 9 by a similar method. Thereafter, the selenium doped GaAs layer 2 is patterned by etching, leaving regions that require diffusion of impurities as shown in FIG. 3(a). The sample in this state is inserted into a quartz tube together with arsenic and an annealing is carried out. Arsenic is included in the quartz tube to prevent deterioration of material due to dissociation of As from the GaAs crystal by applying As pressure to the sample. For example, by carrying out annealing for about 14 hours at 850.degree. C., diffusion of selenium to about a 1 micron depth from the selenium doped GaAs layer 2 takes place and regions 10 where selenium is diffused are produced as shown in FIG. 3(b). These regions are n type and superlattice layer 7 is disordered by the diffusion of selenium.
The prior art method of diffusing selenium from a solid phase source into group III-V compound semiconductors utilizes a selenium doped semiconductors layer as a diffusion source. In this method, however, the semiconductor layer serving as a diffusion source is required to be epitaxially grown, lattice-matching with the semiconductor layer on which the source is grown. Further the amount of selenium diffused depends on the kind and the growth condition of the semiconductor layer. The amount of selenium in the layer serving as a diffusion source is likely to be limited. This limitation makes it difficult to carry out diffusion or makes it impossible to disorder the superlattice when the amount of selenium is insufficient.
For example, doping of selenium into InP series material is difficult and solid-phase diffusion of selenium utilizing the above-described method can not be realized in InP series material.
As other examples of the n type diffusion using selenium according to the prior art, there are those recited in Japanese Patent Publication Nos. 60-24580 and 60-3772.
In these prior arts, amorphous chalcogenide layers including Se are used as a diffusion source. Therefore, there arises no problem in lattice-mismatching. However, these layers require quite complicated processes for producing a patterned diffusion source film. Disposing an Ag layer on the chalcogenide layer, diffusing Ag into the chalcogenide layer in a patterned configuration, removing a portion of Ag and the chalcogenide layer to leave the pattern, and covering the pattern with a heat resistant layer are required.